It is desirable to enhance the strength of articles molded from polyethylene terephthalate resin by employing a reinforcing material, such as glass fibers, asbestos fibers or other fibrous mineral materials, preferably with a coupling agent, in the resin. Furthermore, it is sometimes sufficient to merely increase the modulus by use of a filler, such as beads or a mineral of low aspect ratio, in the resin. However, heretofore it was necessary to use very high mold temperatures, on the order of 130.degree. C. to obtain molded articles from such reinforced or filled resins which had a glossy surface and which were not rough in texture. If these high mold temperatures were not employed, the molded articles had a rough surface with poor gloss. It is believed that below about 130.degree. C., the crystallization rate of polyethylene terephthalate is too slow to result in a molded article having good surface characteristics.
While good surface characteristics can be obtained at molding temperatures of 130.degree. C. or more, the use of such temperatures is not practical in the molding field, for most molds are heated with water and attain temperatures of only about 85.degree.-110.degree. C.
A few molding devices employ heating means such as oil to reach temperatures higher than 85.degree.-110.degree. C. but these generally are inconvenient to use and still either do not generally reach such temperatures or reach them unevenly because of inadequate coring. Because of these heating problems, it has proven commercially unattractive to employ these high temperature molding devices with reinforced or filled polyethylene terephthalate resins.
To increase the speed of crystallization of the injection molded polyethylene terephthalate in the mold, it has been proposed at various times to add to the resin finely divided inorganic substances as nucleating agents. By this step the crystallinity and the density of the injection molded articles are increased and therewith the dimensional stability and stability of shape at elevated temperatures are improved. As solid inorganic substances there have been proposed, for example, metal oxides, alkaline earth metal salts, talc powder, glass powder or metals. It has likewise been proposed to increase the speed of crystallization by adding to the resin a mixture of inorganic nucleating agents with specific epoxides. In all of these cases it has been said to be desirable that the inorganic substances should have a particle size of less than two microns if possible.
In all of the above situations, even when filled polyethylene terephthalate resin has been molded at temperatures above 110.degree. C., it has been found that it is necessary, in order to obtain unobjectionable injection molded articles, that the polyester molding composition should contain as little moisture as possible, the art suggesting that such compositions should contain preferably less than 0.02% by weight of water.
In addition to the above-described problems, it has been found that many of the nucleating agents taught as useful in the prior art do not achieve the normally high heat deflection temperature expected of molded polyethylene terephthalate.